The mechanism of homogeneous CO2 reduction by Ni(cyclam): product selectivity, concerted proton-electron transfer and C-O bond cleavage.

Homogeneous CO2 reduction catalyzed by [Ni(I)(cyclam)](+) (cyclam = 1,4,8,11-tetraazacyclotetradecane) exhibits high efficiency and selectivity yielding CO only at a relatively low overpotential. In this work, a density functional theory study of the reaction mechanism is presented. Earlier experiments have revealed that the same reaction occurring on mercury surfaces generates a mixture of CO and formate.

Pulsed electron paramagnetic resonance spectroscopy of (33)S-labeled molybdenum cofactor in catalytically active bioengineered sulfite oxidase.

Molybdenum enzymes contain at least one pyranopterin dithiolate (molybdopterin, MPT) moiety that coordinates Mo through two dithiolate (dithiolene) sulfur atoms. For sulfite oxidase (SO), hyperfine interactions (hfi) and nuclear quadrupole interactions (nqi) of magnetic nuclei (I ≠ 0) near the Mo(V) (d(1)) center have been measured using high-resolution pulsed electron paramagnetic resonance (EPR) methods and interpreted with the help of density functional theory (DFT) calculations. These have provided important insights about the active site structure and the reaction mechanism of the enzyme.

Applications of pulsed EPR spectroscopy to structural studies of sulfite oxidizing enzymes().

Sulfite oxidizing enzymes (SOEs), including sulfite oxidase (SO) and bacterial sulfite dehydrogenase (SDH), catalyze the oxidation of sulfite (SO(3) (2-)) to sulfate (SO(4) (2-)). The active sites of SO and SDH are nearly identical, each having a 5-coordinate, pseudo-square-pyramidal Mo with an axial oxo ligand and three equatorial sulfur donor atoms. One sulfur is from a conserved Cys residue and two are from a pyranopterindithiolene (molybdopterin, MPT) cofactor.

Identity of the exchangeable sulfur-containing ligand at the Mo(V) center of R160Q human sulfite oxidase.

In our previous study of the fatal R160Q mutant of human sulfite oxidase (hSO) at low pH (Astashkin et al. J. Am.

Metal-sulfur valence orbital interaction energies in metal-dithiolene complexes: determination of charge and overlap interaction energies by comparison of core and valence ionization energy shifts.

The electronic interactions between metals and dithiolenes are important in the biological processes of many metalloenzymes as well as in diverse chemical and material applications. Of special note is the ability of the dithiolene ligand to support metal centers in multiple coordination environments and oxidation states. To better understand the nature of metal-dithiolene electronic interactions, new capabilities in gas-phase core photoelectron spectroscopy for molecules with high sublimation temperatures have been developed and applied to a series of molecules of the type Cp(2)M(bdt) (Cp = η(5)-cyclopentadienyl, M = Ti, V, Mo, and bdt = benzenedithiolato).

Structural studies of the molybdenum center of mitochondrial amidoxime reducing component (mARC) by pulsed EPR spectroscopy and 17O-labeling.

Mitochondrial amidoxime reducing components (mARC-1 and mARC-2) represent a novel group of Mo-containing enzymes in eukaryotes. These proteins form the catalytic part of a three-component enzyme complex known to be responsible for the reductive activation of several N-hydroxylated prodrugs. No X-ray crystal structures are available for these enzymes as yet.

Implications for the mechanism of sulfite oxidizing enzymes from pulsed EPR spectroscopy and DFT calculations for "difficult" nuclei.

The catalytic mechanisms of sulfite oxidizing enzymes (SOEs) have been investigated by multi-frequency pulsed EPR measurements of "difficult" magnetic nuclei (35.37Cl, 33S, 17O) associated with the Mo(v) center. Extensive DFT calculations have been used to relate the experimental magnetic resonance parameters of these nuclei to specific active site structures.

Exchangeable oxygens in the vicinity of the molybdenum center of the high-pH form of sulfite oxidase and sulfite dehydrogenase.

The electron spin echo envelope modulation (ESEEM) investigation of the high-pH (hpH) form of sulfite oxidase (SO) and sulfite dehydrogenase (SDH) prepared in buffer enriched with H(2)(17)O reveals the presence of three types of exchangeable oxygen atoms at the molybdenum center. Two of these oxygen atoms belong to the equatorial OH ligand and the axial oxo ligand, and are characterized by (17)O hyperfine interaction (hfi) constants of about 37 MHz and 6 MHz, respectively. The third oxygen has an isotropic hfi constant of 3-4 MHz and likely belongs to a hydroxyl moiety hydrogen-bonded to the equatorial OH ligand.

Direct detection and characterization of chloride in the active site of the low-pH form of sulfite oxidase using electron spin echo envelope modulation spectroscopy, isotopic labeling, and density functional theory calculations.

Electron spin echo envelope modulation (ESEEM) investigations were carried out on samples of the low-pH (lpH) form of vertebrate sulfite oxidase (SO) prepared with (35)Cl- and (37)Cl-enriched buffers, as well as with buffer containing the natural abundance of Cl isotopes. The isotope-related changes observed in the ESEEM spectra provide direct and unequivocal evidence that Cl(-) is located in close proximity to the Mo(V) center of lpH SO. The measured isotropic hyperfine interaction constant of about 4 MHz ((35)Cl) suggests that the Cl(-) ion is either weakly coordinated to Mo(V) at its otherwise vacant axial position, trans to the oxo ligand, or is hydrogen-bonded to the equatorial exchangeable OH ligand.

Structural studies of the molybdenum center of the pathogenic R160Q mutant of human sulfite oxidase by pulsed EPR spectroscopy and 17O and 33S labeling.

Electron paramagnetic resonance (EPR) investigation of the Mo(V) center of the pathogenic R160Q mutant of human sulfite oxidase (hSO) confirms the presence of three distinct species whose relative abundances depend upon pH. Species 1 is exclusively present at pH < or = 6, and remains in significant amounts even at pH 8. Variable-frequency electron spin echo envelope modulation (ESEEM) studies of this species prepared with (33)S-labeled sulfite clearly show the presence of coordinated sulfate, as has previously been found for the "blocked" form of Arabidopsis thaliana at low pH (Astashkin, A.

Direct demonstration of the presence of coordinated sulfate in the reaction pathway of Arabidopsis thaliana sulfite oxidase using 33S labeling and ESEEM spectroscopy.

Sulfite oxidase from Arabidopsis thaliana has been reduced at pH = 6 with sulfite labeled with 33S (nuclear spin I = 3/2), followed by reoxidation by ferricyanide to generate the Mo(V) state of the active center. To obtain information about the hyperfine interaction (hfi) of 33S with Mo(V), continuous-wave electron paramagnetic resonance (EPR) and electron spin echo envelope modulation (ESEEM) experiments have been performed. The interpretation of the EPR and ESEEM spectra was facilitated by a theoretical analysis of the nuclear transition frequencies expected for the situation of the nuclear quadrupole interaction being much stronger than the Zeeman and hyperfine interactions.

Toward modeling the high chloride, low pH form of sulfite oxidase: Ka-band ESEEM of equatorial chloro ligands in oxomolybdenum(V) complexes.

Two oxomolybdenum(V) complexes, (dttd)MoOCl and [(bdt)MoOCl(2)](-) (where dttd=2,3:8,9-dibenzo-1,4,7,10-tetrathiadecane and bdt=1,2-benzenedithiolate), which contain one or two equatorial chloro ligands, respectively, were studied by electron spin echo envelope modulation (ESEEM) spectroscopy in the microwave K(a)-band (approximately 29GHz). The ESEEM amplitude from the chloro ligands in both compounds is significantly greater than that tentatively attributed to chloride in the vicinity of the oxomolybdenum active site in the high chloride, low-pH (lpH) form of sulfite oxidase (SO). Thus, these ESEEM results rule out equatorial coordination of chloride in the enzyme, although the possibility for a weakly bound chloride in the trans axial position or nearby non-coordinated chloride(s) remains for lpH SO in solution.

Delocalized mixed-valence bi- and trinuclear complexes with short Cu-Cu bonds.

Two mixed-valence copper complexes were synthesized with ligands N-(2-pyridylmethyl)acetamide (Hpmac) and N,N'-(2-methyl-2-pyridylpropan-1,3-diyl)bis(acetamide) (H2pp(ac)2). Dimer [Cu2(pmac)2]OTf and trimer [Cu3(pp(ac)2)2].NaOTf both contain fully delocalized, mixed-valence Cu(1.

Synthesis, characterization, and reactivity of new copper(II) complexes of 2-methylthio-N-(2-pyridylmethyl)acetamide.

Copper(II) complexes were prepared with the new N(2)S(thioether) ligand 2-methylthio-N-(2-pyridylmethyl)acetamide (2-HL(N2S)). [Cu(2-L(N2S))Cl(MeOH)], which formed in the presence of excess triethylamine, is a distorted square pyramidal complex containing the ligand with the amide nitrogen deprotonated. The structurally analogous complex, [Cu(2-HL(N2S))Cl(2)], which formed in the absence of triethylamine, contains 2-HL(N2S) in the tautomeric imidic acid form.

TACN-amino acid conjugates and their copperII complexes.

Triazacyclononane (TACN) was coupled to glycine (L(Gly)), alanine (L(Ala)), and phenylalanine (L(Phe)) via standard solution phase peptide coupling techniques. Copper(II) complexes of these new ligand-amino acid conjugates, [(CuL(Gly))(2)](ClO(4))(4) (1), [(CuL(Ala))(2)Cl](ClO(4))(3) (2), and [Cu(2)L(Phe)Cl(4)] (3), were synthesized and characterized. The X-ray crystal structures of 2 and 3 were determined.